(1) Field of the Invention
This invention relates generally to grounding an electrical conduit or cable that passes through an opening in a structural plate member, such as a deck on board ship, having outboard and inboard sides.
(2) Description of the Prior Act
In order to provide a cable through a deck or bulkhead it is generally necessary to both ground the shielding of the cable to the deck or bulkhead, and to assure that this ground connection is adequately sealed against the hostile environment commonly found aboard ship. Typically, iris springs or rings are provided in contact with the cable shielding in order to create a low impedance path from the shield to ground. Illustrative of the prior art employing this approach is the commonly assigned U.S. Pat. No. 4,904,826 to D. S. Dixon entitled "Internal Shield Ground Adapter for Kickpipe Stuffing Tube". Current must travel through these springs and possibly through threaded interfaces before reaching the ground plane afforded by the deck or bulkhead. A threaded gland nut exerts an axial compressive force on the iris springs. Annular packing material is provided in the same general area so that the gland nut axially compresses the packing for a watertight seal. The iris springs are also compressed by the same nut and therefore may not provide an adequate low impedance path from the cable shielding to ground. Should the nut be torqued to the extent required for achieving adequate pressure on the packing to obtain an environmentally effective seal, the springs can be crushed and the cable shielding will not be grounded to the extent required.
Another aspect of the prior art is illustrated by the Federal Republic of Germany, Patent 29 32 612, assigned to Siemens, AG, which appears to avoid crushing of a ring shaped cable grounding means by the use of two gland nuts, one on each side of the ground plane. The configuration disclosed therein causes difficulty for access in connection with maintenance, difficulty in installation, and is generally complex. Moreover, that configuration interjects threaded joints between the cable shielding and the ground plane, and therefore, is vulnerable to deterioration by corrosion, especially in a marine environment. Furthermore, threaded joints do not provide a reliable environmental seal.
There is high importance connected with reliably grounding the shielding of a cable to the ground plane in marine, and particularly naval marine, environments. High electromagnetic (EM) energy from sources such as lightning, electromagnetic pulse (EMP) signal systems, microwave systems, and radar systems cause high level currents and voltages to be induced on the exposed cable shields. This energy must be shunted to the ground plane before it propagates to equipment to avoid degradation of equipment performance or equipment damage. Such shunting is usually implemented where a cable passes through a metallic boundary, thus preventing the EM energy from entering the protected space inside the boundary.